Measuring rate of rotation is generally implemented through the use of gyroscopes. Gyroscopes can be fabricated using MEMS (microelectromechanical systems) techniques.
FIG. 1 shows a conventional MEMS gyroscope 100 for measuring rate of rotation. MEMS gyroscope 100 includes a suspension frame 102, springs 104-106, a proof mass 108, and a sense mass 110. Proof mass 108 and sense mass 110 are suspended to suspension frame 102 by springs 104-106. In operation, proof mass 108 is vibrated (or driven), e.g., by electrostatic actuation, along the x-axis and (ideally) sense mass 110 does not move. In response to a rotation about the z-axis, proof mass 108 deflects out of the drive axis (i.e., the x-axis) and exhibits a vibration in an axis (i.e., the y-axis) that is orthogonal to the drive axis. Sense mass 110, in turn, vibrates in concert with proof mass 108 along the orthogonal axis. Thus, in the ideal case, sense mass 110 only moves (or vibrates) in response to rotation of MEMS gyroscope 100.
The orthogonal vibration is caused by Coriolis forces that arise from rotation of MEMS gyroscope 100 about the z-axis and act upon proof mass 108 and sense mass 110. The amplitude of the Coriolis-induced orthogonal vibration (referred to herein as Coriolis signal) is typically sensed as a change in capacitance (or charge) between sense mass 110 and an electrode (not shown) fixedly positioned on suspension frame 102. The change in capacitance is converted to a corresponding voltage or current signal (within a sense mass position sensor), electronically amplified, and output as the measured rate of rotation of MEMS gyroscope 100. The change in capacitance, and thus the Coriolis signal, is generally extremely small (e.g., on the order of 10 atto Farads).
Due to undesirable mechanical coupling between proof mass 108 and sense mass 110 caused by, for example, manufacturing imperfections in conventional MEMS gyroscopes, a large error signal can be present along with the relatively small (desired) Coriolis signal. The large error signal is typically in quadrature phase—i.e., 90 degrees relative—with the Coriolis signal and is commonly known as quadrature error.
Various attempts have been made to compensate for quadrature error by modifying the behavior of one or more mechanical elements that are in quadrature phase with the Coriolis signal. For example, in U.S. Pat. No. 6,445,195, entitled “Drive Feedthrough Nulling System”, quadrature error is compensated by applying a time varying electrostatic force to a portion of mechanical elements of a MEMS gyroscope to cancel undesired motion within the MEMS gyroscope. Because an electrostatic force is used to cancel the undesired motion within the MEMS gyroscope, a voltage of similar magnitude to voltages used to originally vibrate (or drive) a proof mass within the MEMS gyroscope is needed. Such voltages are generally greater than 10V and are relatively expensive in terms of silicon area (and thus product cost) to generate and control.
Another technique for compensating for quadrature error consists of modifying the mechanical properties of the drive and sense mechanical elements of a MEMS gyroscope through a trimming technique as discussed in U.S. Pat. No. 6,571,630, entitled “Dynamically Balanced Microelectromechanical Devices”. In this approach, the mechanical properties of the drive and sense mechanical elements are modified by using lasers to remove small amounts of material in order to correct lithographic or etching imperfections. Such a technique generally requires expensive specialized instrumentation and test structures to trim the drive and sense mechanical elements while at the same time testing the gyroscope. This technique, therefore, is not cost effective for the production of low cost, high volume MEMS gyroscopes.
Yet another technique to compensate for quadrature error includes attempting to cancel the quadrature error after the sensed change in capacitance (or charge) has been converted to a voltage or current signal—i.e., after the C-to-V (capacitance-to-voltage) or C-to-I (capacitance-to-current) circuit and prior to any de-modulation. However, the quadrature error signal can be so large that operation of the C-to-V or C-to-I circuit is severely impacted—that is, the dynamic range of the input of the C-to-V or C-to-I circuit must accommodate the large quadrature error as well as the small Coriolis signal. Thus, more gain is typically required within sensing electronics of a MEMS gyroscope after the point in the electronics at which the quadrature has been compensated for as compared to the amount of gain necessary had the quadrature error not been present and, therefore, the resulting effective signal-to-noise performance of the MEMS gyroscope will be reduced.
Accordingly, what is needed is an improved technique for compensating for quadrature error that avoids a need for large voltages to apply an electrostatic force to mechanical elements and avoids any compromise to sensing electronics within a MEMS gyroscope, and which is further generally cost-effective for production of low cost, high volume MEMS gyroscopes. The present invention addresses such a need.